In video signal transmission systems of the type which compress video data on a block basis as for example systems which utilize discrete cosine transforms, data loss and/or data damage frequently occurs on a block basis (e.g. a 16.times.16 array of pixels). These transmission systems normally do not support enough overhead to correct all errors, but rather depend on error concealment techniques to provide substitute data which closely approximates the lost data. Werner Keesen et. al. in U.S. Pat. No. 4,807,033 issued 21 Feb. 1989, disclose an error concealment system for replacing two dimensional blocks of image data. This system includes apparatus for detecting blocks of lost data and replacing the lost image data in the spatial domain with pixel values generated from surrounding pixels by interpolation. The surrounding pixels are examined to determine the existence of image edges (gradients) and thereafter interpolation is performed to generate a block of pixel values to replace the lost or damaged block of pixel values.
Directional interpolation, as used herein, is the process of generating pixel values by performing interpolation which favors a particular direction. Refer to FIG. 1 where the respective squares or blocks represent respective image areas forming the basis of block based compression. The dots within the respective squares or blocks represent pixel values, in this instance representing white or black image points. The center block which is void of dots represents a damaged block for which substitute pixels are to be generated. If for example, pairs of pixels within the two rows of pixels circumscribing the central block are appropriately examined, existing image gradients may be determined as for example the diagonal white line passing through blocks B1 and B9. Then, based on the gradient information, interpolation is performed in the direction of the dominant detected gradients. In the Figure, the pixels which fall on a given arrow are generated from pixels in the adjacent blocks and occurring along extensions of the respective arrows.
The Keesen et. al apparatus detects image gradient information and for single gradients performs one directional interpolation. If more than one strong gradient is detected, a directional interpolation is performed for each of the strong gradients and the arithmetic sum of the respective directionally interpolated blocks of pixels is utilized as the substitute block.
Summing the respective interpolated blocks of pixels gives respective pixel values equal weight and tends to blur image features. Consider a gray image containing a horizontal white line behind a vertical black line. Vertical interpolation will generate a block of gray pixels including the vertical black line. Horizontal interpolation will generate a block of gray pixels including the horizontal white line. The arithmetic average of these interpolated blocks will be a block of gray pixels including a lighter than black vertical line and a darker than white horizontal line, with a gray area at the intersection of the lines.